Heavy hydrocarbons which may be degraded by this novel process are in particular waxes. Such waxes are hydrocarbons in C.sub.12 to C.sub.50 which generally are saturated. However, they can be sometimes unsaturated. They can be derived from processes of synthesis like Fischer-Tropsch. In fact, their structure is very similar to the structure of oligomers based on low molecular weight polyethylene.
Polymers which may be degraded by this novel process are polymers having chains with carbon--carbon bonding, for example polyethylene, polypropylene or polystyrene.
Among the fields of application of the invention, mention may be made of environmental protection or the polymer production industry, in particular appertaining to polymers with controlled molecular mass, petrochemical products, fuels (hydrocarbon fractions), lubricants or alternatively energy.
The invention is in particular advantageous in the field of fuel for obtaining hydrocarbon fractions like diesel, which is constituted of hydrocarbons in C.sub.12 to C.sub.20, from waxes derived from Fischer-Tropsh's synthesis. As these waxes are solid at room temperature, their conversion needs very high temperatures with conventional catalysts. According to the present invention this conversion can be achieved by reacting waxes with hydrogen in less severe conditions.
The invention can also find applications in the food packaging industry, or that appertaining to any other type of packaging where the polymer waste produced during manufacture cannot be reused.
Heightened environmental protection requires the implementation of reliable new techniques for processing the polymer waste commonly referred to as plastics. Among the various methods envisaged for the reuse or disposal of used polymers, energy and/or chemical recycling may afford satisfactory solutions complementary to material recycling.
Among the solutions relating to plastics waste, incineration with or without energy recovery constitutes one possible way, but the temperatures needed for this type of processing are very high, of the order of 400 to 700.degree. C. However, this type of process can only lead to energy recovery from polymers, and has the drawback of being unecological because it would increase the greenhouse effect.
Chemical recycling is starting to attract great interest. This technique consists in chemically decomposing the macromolecules with a view to obtaining molecules which have low molecular masses and can be reused with regard to their physical or chemical or energy (fuel) properties. In general, catalytic degradation (cracking or hydrocracking) of polyolefins makes it possible to improve the selectivity of this type of reaction in comparison with purely chemical and thermal processes, and consequently to obtain directly hydrocarbon fractions which are easier to upgrade. The catalysts for cracking and hydrocracking polypropylene or polyethylene are generally silicas, silica/aluminas, zeolites or catalysts based on cobalt and molybdenum which operate at elevated temperature, typically between 200 and 600.degree. C. and most often towards 400.degree. C. and under high hydrogen pressure. These techniques are described, in particular, by BELTRAME et al., in Polymer degradation and stability, 26, (1989) 29-220., HIROTA et al., in Makromol. Chem. Macromol. Symp. 57, 161-173 (1992), Y. ISHIRA et al., Fuel, 69, 978, (1990), I. KORFF et al., in Erdol Erdgas 105, (1989) 223.